Abstract

This paper presents new data obtained from the field, 34 whole-rock silicate analyses, and age dates, in order to provide evidence for a late-stage, water-enriched, igneous origin for the orbicular rocks of the Sandia Mountains in New Mexico. A recent paper by Thompson and Giles (1974) proposed a metasomatic origin for these orbicules which resulted from reaction of the Sandia granite with a large biotite monzonite-diorite xenolith (see p. 912). Our data, collected since 1972 without knowledge of the work of Thompson and Giles, indicate that the biotite monzonite is not a xenolith but rather a product of a magmatic event during the late stages of the Sandia granite formation. In 197.1 (abs.), Daugherty and Asquith suggested an igneous origin in which the orbicules were formed by alternating precipitation of biotite and plagioclase in a convecting magma, one phase being precipitated in a hotter region and the other in a cooler part of the magma. Phase relationships can be used to argue against this theory since, two different phases could not be precipitating in different parts of a magma simultaneously at different temperatures. During the circulation of an orbicule from a cooler to a hotter (or vice versa) region of a magma, a cotectic boundary must be crossed, which is impossible to do. Kelley and Northrop (1975) in their memoir on the Sandia Mountains reviewed briefly the occurrence of the orbicular rocks and, without new data, concluded that a metasomatic origin as proposed by Thompson, and Giles (1974) was unacceptable and that an early magmatic origin was most likely. Using zircon distribution patterns, Held and Harris (1978, abs.) interpreted an igneous origin from one specimen of the Sandia orbicular rock. We believe that our new data will help resolve this controversy.

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